The present invention relates to a fuel system for a motor vehicle.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Hybrid vehicles and in particular so-called plug-in hybrids that are equipped with an internal combustion engine and an electric machine and in which electro-motoric operation predominates, are often equipped with a fuel tank, which is configured as a pressure tank, in which an operating pressure is present which is higher than the ambient pressure. The increased operating pressure enables reducing outgassing of hydrocarbon vapors from the liquid fuel into the tank. As a result the activated carbon filter is loaded to a lesser degree with hydrocarbon vapors and thus does not have to be regenerated as often, which is only possible during operation of the internal combustion engine. Prior to refueling of such a pressure tank, however, the operating pressure has to be reduced so that the tank pressure is only slightly above ambient pressure. Only then a tank cover, which closes the filling tube, can be opened and the fuel tank can be filled with fuel. For this purpose a tank cover opener or control switch, whose actuation indicates an intent to refuel, can be provided in the motor vehicle. The control then initiates a pressure decrease in the fuel tank by opening the tank isolation valve (FTIV), which is a part of a tank ventilation device of the fuel tank. The pressure decrease in the fuel tank is detected by a pressure sensor and is transmitted to the control device. After the pressure compensation has occurred or when the pressure in the fuel tank has almost reached ambient pressure, and with this the readiness for refueling is established, the control device unlocks a tank flap behind which the tank cover is arranged. This prevents the tank cover from being opened when there is still an increased pressure in the fuel tank.
In motor vehicles of the applicant with hybrid drive, currently two different variants of fuel systems with a pressure tank, a tank isolation valve and an activated carbon filter are used. In the first variant, the tank isolation valve is arranged above the fuel tank and communicates via a liquid separator (liquid trap) within the fuel tank with a quick-venting valve in the head- or gas space of the fuel tank, which head- or gas space is also connected with the filling tube through a refueling venting line and a compensation container. In this variant the venting of the fuel tank occurs in a different manner during operation than during refueling of the fuel tank. While during operation, hydrocarbons vapors, which are generated due to an increase of the ambient temperature and the resulting evaporation of fuel, are discharged from the fuel tank through the quick-venting valve, the liquid separator, the tank isolation valve and the downstream arranged activated carbon filter, the hydrocarbon vapors displaced during refueling from the head- or gas space of the fuel tank are conducted through the refueling-venting line back into the filling tube, where they are suctioned off by means of the gas return of the fuel nozzle. As a result of the separation of the operating-venting and refueling-venting in this first variant, the quick-venting valve has to be arranged above the switch-off level of the fuel tank. In order to prevent overfilling of the fuel tank through the quick-venting valve, the tank isolation valve therefore has to be closed during refueling. This enables preventing loading of the activated carbon filter with hydrocarbon vapors during refueling, which in hybrid vehicles has the advantage that the activated carbon filter has to be regenerated less often. However, in this variant hot ambient conditions may lead to a spontaneous pressure increase in the fuel tank when the tank isolation valve is closed for the refueling process prior to opening the tank cover. When the tank flap is already unlocked, this may lead to pressurized hydrocarbon vapors in the fuel tank being abruptly discharged when opening the tank cover and being blown into the face of the driver.
In the second variant, the tank isolation valve and the activated carbon filter are connected to the filling tube, which has a separator function, while the quick-venting valve serves as shutoff valve for limiting the fuel level. In this variant the fuel tank is vented during operation and also during refueling by way of the tank isolation valve. Therefore the tank isolation valve has to be open during refueling so that the hydrocarbon vapors that are displaced from the head- or gas space can reach the downstream arranged activated carbon filter for being absorbed. Because the fuel tank is vented during refueling in the manner of a vapor recovery via the head of the filling tube, overfilling can be prevented. However, because the filling tube serves as liquid separator, carryover of liquid fuel through the open tank isolation valve into the activated carbon filter during refueling cannot be fully avoided in this variant.
It would therefore be desirable and advantageous to provide an improved fuel system to overcome the problems of the two variants by way of a control.